Why basic mathematical skills need tremendous improvements?

The other day I went to a local shop in my locality to buy grocery items.

I found the owner who was just in his early twenties was using a calculator for basic addition. For ex- he was calculating 20 + 30 on his calculator.

Out of curiosity, I asked him why he uses a calculator for such basic calculations. I am sure hearing his answer you will simply make fun of him. His answer was, "I know 20 +30 is 50 but sometimes it could be different"

Now, you suggest what should be done to these young people.

He badly missed his basic mathematical lessons, I believe. This is not a single case but entire generation is facing this problem.